Anharmonicity-driven low lattice thermal conductivity and high thermoelectric response in monolayer CeX2 (X = O, S, Se, Te).

Rare-earth materials drive technological innovations from clean energy to quantum devices, yet their two-dimensional (2D) derivatives remain largely unexplored. Here, we decode the thermal and electronic transport mechanisms in 2D rare-earth dichalcogenides CeX2 (X = O, S, Se, Te) through first-principles calculations, temperature-dependent effective potentials theory, anharmonic lattice dynamics, and Boltzmann transport theory. Both ab initio molecular dynamics simulations and finite-temperature phonon spectra demonstrate that CeX2 compounds are structurally stable. In this class of materials, three-phonon scattering dominates lattice thermal transport. The lattice thermal conductivity (kL) and bandgap decrease with increasing atomic mass of the X element. At 300 K, CeTe2 exhibits an ultralow kL of 4.26 W m-1 K-1, approximately twenty times smaller than that of MoS2. Electronic structure analysis reveals that the conduction band minimum of CeX2 is dominated by localized Ce 4f orbitals, forming a flat band. The valence band maximum consists of hybridized f (Ce) and p (X) orbitals, enabling both high electrical conductivity and a large Seebeck coefficient. Under p-type doping, the power factor can exceed 250 µW m-1 K-2. This work not only expands the family of viable thermoelectric materials, but also provides a comprehensive theoretical framework for engineering quantum-confined rare-earth architectures with tailored energy conversion functionalities.